1. Field of the Invention
The present invention relates to a structure of a semiconductor device having an active region of a nitride semiconductor on a substrate and a manufacturing method of the semiconductor device.
2. Description of the Related Art
As a semiconductor device using a compound semiconductor, particularly, a high output high frequency semiconductor device, an HEMT (High Electron Mobility Transistor) using GaN can be taken as an example. A schematic cross-sectional structure of an HEMT device 90 is illustrated in FIG. 6. In FIG. 6, a p-type GaN layer (conductive semiconductor layer) 92, a channel layer 93 and an electron supply layer 94 are formed by epitaxial growth on a substrate 91. The channel layer 93 serving as an active layer in HEMT operation is formed of semi-insulating (non-doped) GaN, and electron supply layer 94 serving as an active layer in HEMT operation is formed of n-AlGaN (exactly, n-type Al0.20Ga0.80N for example). A two-dimensional electron gas layer is formed on the channel layer 93 side of the interface between the channel layer 93 and electron supply layer 94. The two-dimensional electron gas layer is formed between a source electrode 95 and drain electrode 96 to allow current to flow between the source and drain electrodes 95 and 96. ON/OFF of the two-dimensional electron gas channel is controlled by voltage applied to a gate electrode 97, whereby switching operation is performed. At this time, the speed (mobility) of the electron in the two-dimensional electron gas becomes extremely high, thereby allowing high-speed operation. Further, since the GaN has a larger band gap than that of GaAs, etc., the HEMT device 90 exhibits a high breakdown voltage and can perform high output operation. In order to obtain favorable amplification characteristics or switching characteristics in this configuration, it is necessary to increase on/off ratio of current flowing between the source and drain electrodes 95 and 96 or on/off ratio of a resistance therebetween. Note that FIG. 6 illustrates the simplest structure of the HEMT device, and the actual structure thereof often differs from that of FIG. 6, wherein, for example, the shape of a contact between source electrode 95 and electron supply layer 94, shape of a contact between the drain electrode 96 and electron supply layer 94, and shape around the gate electrode 97 are actually more optimized than illustrated.
The characteristics of the HEMT device 90 are significantly influenced by the crystallinities of the channel layer 93 and electron supply layer 94 serving as the active layer, and the crystallinities and manufacturing costs of the channel layer 93 and electron supply layer 94 depend strongly on the substrate 91, so that the selection of the material of the substrate 91 is an important factor. For example, the substrate 91 may be an insulating material, such as a sapphire substrate or a semi-insulating SiC substrate. In recent years, as a GaN wafer, an n-GaN (n-type GaN) wafer of a manageable size can be obtained at low cost for use as the substrate 91. In this case, conductivity of the substrate 91 is high.
In the case where a highly conductive substrate is used as the substrate 91, when the entire substrate 91 is electrically connected to the source electrode 95, the need of forming a source electrode pad on the front surface (upper surface) side of the HEMT device 90 is eliminated, which is advantageous in terms of layout and which further produces an advantage of reducing on-resistance. And, there is known current collapse which may occur in a GaN-based HEMT device. The current collapse is a phenomenon in which current is reduced at the time of switching from OFF to ON during large current operation time. It is known that setting the conductive substrate 91 side to a constant potential so as to suppress electric field concentration is effective also from a viewpoint of suppressing the current collapse. On the other hand, however, the use of the highly conductive substrate produces a disadvantage that a leak is caused between the source electrode 95 and drain electrode 96 through the substrate 91 to degrade breakdown characteristics between the source electrode 95 and drain electrode 96. In order to improve this, in the structure of FIG. 6, the p-type GaN layer 92 is inserted between the n-GaN (substrate 91) and channel layer 93. In this case, the conductive substrate 91 is biased by a p-n junction between the substrate 91 and GaN layer 92, so that a leak through the substrate 91 is suppressed. However, there may be a case where positive holes in the p-type GaN layer 92 adversely affect the operation (gate current or drain current) of the HEMT device 90. To suppress this, a configuration in which an n-type layer and a p-type layer are further inserted between the p-type GaN layer 92 and channel layer 93 is proposed in Patent Document 1.
In the case where the insulating material such as sapphire, a leak through the substrate 91 does not occur; however, the source electrode configuration as described above cannot be achieved and, accordingly, the above advantage cannot be obtained. Thus, also in this case, the conductive semiconductor layer (p-type GaN layer 92) is inserted between the insulating substrate 91 and channel layer 93. With this configuration, the same advantage as in the case where the conductive substrate 91 is used can be obtained.
As a result, as described above, to form the p-type GaN layer 92 on the substrate 91 is effective irrespective of the type of the substrate 91.